Method and device for adapting a threshold value of a detection device

ABSTRACT

A method for adapting a threshold value of a detection device comprising the following steps: Transmission of a signal burst having a predefined pulse-repetition interval, a predefined burst length and a defined signal frequency; receiving and processing of a receive signal, in particular by means of amplification, filtering and demodulation, so as to obtain the envelope; determining a receive-signal magnitude as interference-level sample from the receive signal following a first predefined time duration (Δt 1 ) starting with a signal burst within a predefined second time duration (Δt 2 ), preferably at the end of the pulse repetition interval; and adapting the threshold value of the detection device as a function of the interference level sample. Also provided is a device for adapting a threshold value of a detection device.

FIELD OF THE INVENTION

The present invention relates to a method and a device for adapting athreshold value of a detection device and, in particular, to a methodand a device for adapting a threshold value, on the basis of anultrasonic system for detecting the surroundings of a vehicle.

BACKGROUND INFORMATION

There is increasing demand for detection devices, especially in theautomotive sector. For detecting the surroundings of a vehicle, the useof ultrasonic systems is preferred.

From German Patent Application No. 196 45 339, an ultrasonic parkingassistance system and a method for a distance measurement as a functionof vehicle data in a motor vehicle are known. This system emits awarning signal if an echo signal that lies inside a time windowestablishing a measuring range exceeds a specific threshold value.According to the German laid-open document, a static threshold valuecharacteristic curve having a predefined number of interpolation pointsis transmitted via a transmitter/receiver line from a control unit to asensor, typically at the beginning of the parking operation. A staticthreshold may be configured under two aspects: First, one that isoptimized with respect to a minimal false alarm by noise andinterference sources, which goes hand in hand with a high thresholdlevel and entails low sensitivity or range of the detection system as afunction of the reflecting size of the object to be detected. Secondly,there is the possibility of providing high sensitivity of the system bysetting the threshold close to the noise/clutter limit, which results ina system exit due to “phantom” obstacles by a temporarily occurringinterference source or by temporary interference influences.

Clutter or ground clutter refers to reflections of an ultrasound signaldue to gravel or crushed stones, for instance. As a consequence, theconfiguration of a static threshold value characteristic curve involvesa compromise between the aspects of high sensitivity, if possible, and astill acceptable false alarm rate under worst-case ambient conditions.Noise is caused by electronic amplification of the reception of thetransducer signal and determines the best-case sensitivity limit or thelowest possible threshold.

The use of an adaptive threshold on the basis of a binary, i.e.,digital, analysis on the level of the control unit is not possibleeither, for two reasons: First of all, the track evaluations in thecontrol unit and a dead time arising in the transmission areincompatible with the latency-time demands of expanded detection-systemfunctions such as autonomous driveaway or detection in the region of theblind spot. On the other hand, the transmitted time-analog,binary-valued information from the sensor, i.e., thetransmitter/receiver unit, to the control unit, includes only a radialobstacle or clutter distance, but no quantization of the magnitude orthe amount of the clutter or noise level. No suitable adaptive thresholdadaptation in the control unit is possible on this basis, since nodifferentiation criterion between a target object and clutter, i.e.,interference reflections, is provided.

However, the expanded functions of the detection system should also beutilizable in a dynamic environment in which the acoustic interferencelevel may change rapidly, for instance due to passing interferences suchas two-stroke motorcycles, wind noise and/or so-called microphonics,which depend on the engine load or engine speed or other vehiclevibrations.

SUMMARY OF THE INVENTION

In contrast to the known design approach, the method for adapting athreshold value of a detection device according to the present inventionand the device according to the present invention have the advantage ofproviding an adaptive threshold control in which the system varies thedetection threshold as a function of external conditions. This allows adifferentiation between an obstacle and interference signals, so thatboth a minimal false alarm, caused by interference sources, and highsensitivity of the system are able to be ensured.

In essence, the present invention is based on the idea of implementingan interference-level measurement during a time interval in which noreflected measuring signal is expected. This allows an adaptivethreshold regulation whose detection threshold or actuating variable isdetected within a time range—preferably at the end of a measuringcycle—during which no reflected useful signal with a correspondinglyhigh signal level occurs. This makes use of the characteristic that mostexternal interferences (wind noise, passing motor vehicles,microphonics) occur at a constant level across the entire measuringcycle, whereas useful signals from target objects become miniscule atthe end of a measuring cycle due to the object distance related to thepropagation delay and the attendant distance attenuation, whichincreases quadratically.

Furthermore, useful signals from reflecting objects have thecharacteristic of a peak on a demodulated receive signal whosecharacteristic width is a function of the width of the transmissionburst. In contrast, the profile of the interference level is virtuallyconstant. Useful target and constant interferences are thusdifferentiable on the basis of their time constant.

In other words, a method for adapting a threshold value of a detectiondevice is provided, which encompasses the following steps: Transmittinga signal burst with a predefined pulse-repetition interval, a predefinedburst length and a defined signal frequency; receiving and processing areceive signal, in particular by means of amplification, filtering anddemodulation, so as to obtain the envelope; determining a receive-signalmagnitude from the receive signal as interference-level sample followinga first predefined time interval, starting with a signal burst, within apredefined second time duration, preferably at the end of the pulserepetition interval; and adapting a threshold value of the detectiondevice as a function of the interference level sample.

According to a preferred further development, the interference levelsample is ascertained from the envelope of the time-windowed receivesignal at the end of the pulse-repetition interval by means of apeak-value rectifier, the peak-value rectifier preferably having ahold-time constant that is greater than the pulse-repetition intervalPRI. Use of a peak-value rectifier advantageously allows the maximum ofthe envelope of the time-windowed receive signal to be ascertained andstored for a predefined time in a simple manner. By selecting anappropriate hold-time constant it is possible to maintain the maximumreceive-signal level of the noise or interference level sample,ascertained according to the present invention, across the measuringcycle in a largely constant manner for a predetermined time. Thishold-time constant of the peak-value rectifier determines the reactivityof the adaptation in response to temporally changing ambient conditions.

According to another preferred further development, the peak-valuerectifier is configured such that its load-time constant exceeds theburst length, preferably that its hold-time constant is longer than thepulse-repetition rate and, in particular, that the temporal acquisitionduration of the interference level samples is longer than the load-timeconstant. As a result, the noise-sample determination is unable tosimulate the profile of a useful signal, i.e., a receive peak caused byan object reflection, or it may simulate it to a limited extent only. Incontrast, the load-time constant is small relative to the timecharacteristic of the dynamics of the interference level (for instance,as a function of interference sources driving past) and is easily ableto follow it within the selected time-window of the noise sample. Ittherefore results that the length of the time window of the noise samplemust be greater than the load-time constant so that the peak-valuerectifier is able to recharge to the maximum of the magnitude of thereceive-signal envelope within this interval.

According to another preferred further development, a timer stipulatesthe first predefined time duration, preferably 35 ms to 55 ms, which istriggered by the predefined clock pulse, preferably at a pulse frequencyof 15 Hz to 30 Hz. This has the advantage that the timer component,which is preferably triggered by a transmit request, releases the inputof a detection device, preferably the peak-value rectifier, after apredefined period following the transmit trigger, i.e., following thetransmission of a signal pulse, so as to detect the receive-signalmaximum, and then locks it again by a subsequent transmit trigger. In asimple and cost-effective manner, the timer may preferably be realizedby a monoflop.

According to another preferred further development, a predefinedthreshold value is added to the ascertained noise sample, the predefinedthreshold value preferably having the time characteristic of a VZ1element and/or programmed interpolation points within the meaning of astatic characteristic curve. This advantageously provides a dynamicoutput-threshold value, which includes both a static and a dynamiccomponent as a function of the receive-signal magnitude. The staticportion is used to intercept the magnification of the interference levelcaused by ground clutter, which occurs in a localized and thustime-dependent manner depending on the sensor installation height andangle of incidence, in a typical distance range of 1 to 2 m, or within atime window of approximately 6 to 12 ms.

According to another preferred further development, the receive signal,having been amplified first, is filtered by a bandpass filter,preferably with a mid-frequency on the resonant frequency or transmitfrequency of a transducer and with a pass-bandwidth that is proportionalto the inverse value of burst length Δt₁, and the envelope isdemodulated, especially by means of a double-way rectifier and alow-pass filter, prior to the noise sample being ascertained therefrom.The threshold ascertained from the dynamic and static portion ispreferably also applied to this envelope. This advantageously providesan easily implementable and effectively working signal processing in thedetection device, thereby circumventing the necessity of amirror-inverted threshold to be applied to the bipolar receive signalwith carrier.

According to another preferred further development, signal processing isimplemented by an I/Q demodulation, cross-correlation of an I and Qchannel with an ideal time-shifted and, in particular, alsoDoppler-shifted replica of the transmit signal, and the amount is formedfrom the sum of the squares of the cross-correlations of the I and Qchannel, the amount being scaled with the auto-correlation of thereplica. This scaled amount, as output signal of the optimal receiver,corresponds to the output of the low pass after the double-passrectifier in the above simple realization.

According to another preferred further development, the transmit/receivedevice includes a transducer for emitting signal pulses at a signalfrequency of between 40 KHz and 100 KHz, preferably with a pulse lengthof 150 μs to 500 μs, and, in particular, a pulse-repetition rate of 10Hz to 30 Hz, and for receiving an analog acoustic receive signal. Thisadvantageously provides an ultrasound-based system having adaptivethreshold adjustment, which may be used in the automotive sector, inparticular.

According to another preferred further development, an addition devicefor adding the receive-signal magnitude, as interference level sample,to a predefined threshold value, which preferably has a predefined timecharacteristic that is, in particular, acted upon by thepulse-repetition rate, to form a resulting dynamic threshold value.

According to another preferred further development, a comparator isprovided to generate a binary-valued detection signal by comparing thedemodulated receive signal to the dynamic threshold value. This has theadvantage of realizing a detection device having a dynamic detectionthreshold value, which is able to be implemented in a cost-effectivemanner, a binary value (obstacle present or no obstacle present) beingoutput in a time-analog manner.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic block diagram of a detection device having anadaptive detection threshold value, to elucidate a specific embodimentof the present invention.

FIG. 2 a is a first individual schematic signal diagram over time tafter various signal-processing steps.

FIG. 2 b is a second individual schematic signal diagram over time tafter various signal-processing steps.

FIG. 2 c is a third individual schematic signal diagram over time tafter various signal-processing steps.

DETAILED DESCRIPTION

In the figures, identical numerals denote the same or functionallyequivalent component parts. FIG. 1 schematically shows a block diagramin which a pulse-repetition clock 10 (pulse repetition interval PRI) istransmitted to a signal-modification device 11. Signal-modificationdevice 11 modifies, in particular, the signal form and the signalduration, preferably into a square-wave pulse having a width Δt ofbetween 100 μs and 1 ms, in particular, 300 μs. Signal-modificationdevice 11, especially a pulse shaper, determines Δt and, according to analternative exemplary embodiment, may realize another pulse shape asenvelope, for instance a Gauss function Q(t)=exp(−π(t/Δt)²), instead ofa square-wave pulse.

Signal-modification device 11 routes carrier signal 110 from a transmitgenerator 100 via a switching device 111 or, in the case of thealternative exemplary embodiment, via an AM modulator 111′. Transmitgenerator 100 is operated at a predefined frequency, preferably in arange between 40 KHz and 100 KHz, in particular approximately 50 KHz.Switching device 111 or AM modulator 111′, triggered bysignal-modification device 11, triggers a transmit/receive device 13,preferably an acoustic transducer, in transmit mode via a signal pulse112 to be transmitted. Clock pulse 10 has a pulse-repetition rate, or apulse repetition interval (PRI), of between 35 ms and 60 ms, preferablyapproximately 50 ms. Transmit or receive device 13, triggered by signalpulse 112, outputs a transmit signal 14 at the predefined frequency,which has a corresponding envelope according to the alternativeexemplary embodiment.

At the same time, transmit/receive device 13 receives a receive signal15, which is preferably transmitted to an amplification device 16. Theamplification of analog receive signal 15 in amplification device 16 isfollowed by filtering with the aid of a filter device 17, preferably abandpass filter, which, in particular, includes a mid-frequency thatcorresponds to the signal frequency of transmit pulse 14 and, inparticular, has a pass bandwidth that is proportional to the inversevalue of burst length 1/Δt. Appropriately filtered receive signal 17′ isthen supplied to a double-way rectifier 18 having integrated low pass19. Filtered receive signal 17′ is preferably demodulated with respectto its amplitude in this non-coherent demodulation device 20, which ismade up of double-way rectifier 18 and lowpass filter 19. In this way,filtered receive signal 17′ becomes a demodulated receive signal 21,which corresponds to the envelope of filtered receive signal 17′, i.e.,the carrier signal. In double-way rectifier 18 a signal having onlypositive half waves is obtained, the low-pass filtering in low passfilter 19 filtering out high-frequency spectrums.

Via a switching device 22, preferably a field effect transistor,demodulated receive signal 21 is then sent to an amplitude detectiondevice 23, preferably a peak-value rectifier. Switching device 22 istriggered by clock pulse 10, which, however, implements a delay having apredefined time duration, for instance, 45 ms, via a timer 24 such as adead time element, in particular. The switching device thereuponswitches demodulated receive signal 21 through to amplitude detectiondevice 23 in accordance with delayed clock pulse 10′, until nextnon-delayed clock pulse 10, as reset, opens switching device 22 again.In this way a receive-signal magnitude is identified by amplitudedetection device 23, preferably by the peak-value rectifier, during asecond predefined time interval following a first predefined timeinterval after a clock pulse, preferably by the time the following clockpulse occurs.

For the purposes of an optimal receiver, signal processing may also becarried out in a more involved fashion.

To this end, the following processing steps are conceivable, which maybe realized both in hardware and in software provided receive signal 15is sampled sufficiently:

-   -   a) I/Q demodulation;    -   b) cross correlation of the I and Q channel by means of ideal        time-shifted and possibly also Doppler-shifted replica of the        transmit signal;    -   c) amount formation from the sum of the squares of the cross        correlation of the I and Q channel;    -   d) scaling of the amount with the auto correlation of the        replica.

This scaled amount, as output signal of the optimal receiver,corresponds to output signal 21 of the low pass after double-wayrectifier 18 in the above simple realization.

If an ultrasound ambient environment detection device is assumed andgiven a sound velocity in air of approximately 330 m/s, a delay of 45ms, for instance, effected by timer 24, has the result that amplitudedetection device 23 detects the maximal signal level of a signal samplein the distance range of approximately 7.7 to 8.5 m. If one assumes thatthis amplitude value which occurs in this predefined time window iscaused by an interference source and has not been generated by atransmit pulse reflected at an obstacle, it is possible to derive anadaptive detection threshold as interference level sample 25 from thisreceive-signal magnitude.

The sonic field intensity of a reflected transmitted pulse is generallynot sufficient to generate a clearly detectable receive signal fortypical obstacles or targets having distances of >7 m. If required, itis also possible to lower the repetition rate of clock pulse 10 from 20Hz to 16 Hz, for instance, and to correspondingly increase the timedelay by timer 24, from 45 ms to 55 ms, for example, which furtherdelays the noise sample and would mean a correspondingly greaterdistance range. Furthermore, the selection of the load-time constantensures that useful signals from object reflections will contribute onlynegligibly to the loading of the peak value, i.e., interference level.

Amplitude detection device 23, preferably the peak-value rectifier, hasa time constant that amounts to 240 ms, in particular, and thus isnearly five times higher than pulse repetition rate 10, i.e., the clockpulse, thereby keeping the receive signal magnitude, as interferencelevel sample 25, i.e., the maximum noise sample level, largely constantacross the measuring cycle following the opening of switching device 22.This time constant of amplitude detection device 23 determines thereactivity of the adaptation to dynamically changing ambient noises, forinstance. Peak-value rectifier 23 is configured such, in particular,that its load-time constant t_(charge) exceeds burst length Δt, it beingpreferred that its hold-time constant t_(hold) be longer thanpulse-repetition rate PRI, and, in particular, that the temporalacquisition duration of the interference level sample T_(sample) belonger than load-time constant t_(charge). Consequently, thenoise-sample determination is unable to simulate the characteristic of auseful signal, i.e., a receive peak caused by an object reflection, orit is able to simulate it to a limited extent only. However, load-timeconstant t_(charge) is small relative to the time characteristic of thedynamics of the interference level (for instance as a function ofinterference sources driving by) and is easily able to follow it withinselected time-window t_(sample) of the noise sample. It thus followsthat the length of time window T_(sample) of the noise sample must begreater than load-time constant t_(charge) in order to ensure thatpeak-value rectifier 23, within this interval, is able to recharge tothe maximum of the magnitude of the receive-signal envelope(t_(hold)>PRI, t_(charge)>Δt, T_(sample)>t_(charge)). A configurationexample is: PRI=50 ms, Δt=300 μs, t_(charge)=5 ms, t_(hold)=300 ms,T_(sample)=8 ms (between 42 . . . 50 ms after transmission of a burst ineach case).

Clock pulse 10 also goes to a static threshold-value generation device26 and triggers it. In a simple realization, static threshold-valuegeneration device 26 preferably has the time characteristic of a VZ1element, which is activated at each trigger or clock pulse. Furthermore,as an alternative to the time characteristic of the VZ1 delay element,it is possible, for instance, to predefine a particular timecharacteristic of the static threshold via programmed interpolationpoints. Noise sample 25 is added to a static threshold value 27 via anaddition element +. This results in a static threshold value 27, raisedby the receive-signal magnitude as interference level sample 25, whichis illustrated in block 28 according to FIG. 1.

Demodulated receive signal 21 and an adaptive threshold value 29,obtained from adding static threshold 27 and interference level sample25, will then be compared in a comparator 30 as comparison device. Thecomparison by comparator 30 yields a binary, i.e., digital, detectionsignal 31, which has a high level if demodulated receive signal 21 has alevel that is above adaptive threshold value 29. Dynamic threshold 29,which determines the offset of the receive-signal amplitude asinterference level sample 25 as a function of the noise sample suppliedvia switching device 22, thus allows the sensitivity of the detectiondevice to be adapted as a function of occurring interference sources orinterference variables. The receive-signal magnitude as interferencelevel sample 25 is preferably raised by a level of 10 dB bymultiplication by a fixed value. This corresponds to a false alarm rateof approximately 4.5E-5 in the case of white noise.

FIG. 2 a, by way of example, shows a receive signal 15, received fromtransmit/receive device 13, over time t. An amplitude maximum in theregion of the ordinate, which is generated by the transmission burst orsignal pulse 12, is attenuated exponentially, whereupon, according toFIG. 2 a, an amplitude rise occurs in the middle range between the twosignal pulses 32, which is caused by a reflected signal pulse 33. Theimage is repeated in accordance with clock pulse 10. Time duration Δt₁between the appearance of transmitted signal pulse 32 and receivedsignal pulse 33 as time duration, when multiplied by the sonic velocityin air, results in the distance to an obstacle.

FIG. 2 b shows demodulated receive signal 21 according to FIG. 1. Whencompared to FIG. 2 a, it becomes clear that receive signal 15 isfiltered and provided only with positive signal values insignal-processing device 16, 17, 18 and 19 according to FIG. 1. Duringtime period Δt₂, switching device 22 according to FIG. 1 is switchedthrough, and amplitude-detection device 23 acquires aninterference-level sample 25. FIG. 2 c schematically illustrates acorresponding dynamic threshold 29 according to FIG. 1.

Although the method according to the present invention is described onthe basis of a preferred exemplary embodiment, it is not limitedthereto, but is able to be modified in a plurality of ways. Forinstance, in addition to ultrasonic detection devices, other detectiondevices such as in the radar spectrum may also be provided with adynamic threshold value according to the present invention. Thementioned time durations or frequency values should therefore beconsidered examples only and may also be in the ns-range or GHz range,for instance. Although described with reference to a surroundingsdetection of a vehicle, in particular a motor vehicle, the presentinvention is applicable to any other detection device.

1. A method for adapting a threshold value of a detection device,comprising: transmitting a signal burst at a predefined pulse-repetitioninterval, a defined burst length, and a defined signal frequency;receiving and processing a receive signal to obtain an envelope;ascertaining a magnitude of the receive-signal as an interference-levelsample from the envelope of the receive signal after a first predefinedtime duration beginning with the signal burst, and within a predefinedsecond time duration prior to a next signal burst; generating a dynamicthreshold value by summing the interference-level sample with a staticthreshold value having a predefined time characteristic; comparing theenvelope of the receive signal to the dynamic threshold value; andgenerating an output indicating that an object has been detected whenthe envelope exceeds the dynamic threshold value.
 2. The method asrecited in claim 1, wherein: the receive signal is amplified, filtered,and demodulated to obtain the envelope.
 3. The method as recited inclaim 1, wherein: the ascertaining is performed at an end of thepulse-repetition interval.
 4. The method as recited in claim 1, wherein:the ascertaining is performed from an envelope of the receive signal bya peak-value rectifier, the peak-value rectifier including a hold-timeconstant that is greater than the pulse-repetition interval.
 5. Themethod as recited in claim 4, further comprising: configuring thepeak-value rectifier so that a load-time constant thereof exceeds thedefined burst length, so that a hold-time constant thereof is longerthan the pulse-repetition interval, and so that a temporal acquisitionduration of the interference-level sample is longer than the load-timeconstant.
 6. The method as recited in claim 1, wherein a timerstipulates the first predefined time duration at 35 ms to 55 ms, andwherein the timer is triggered at the predefined pulse-repetitioninterval, at a clock frequency of 15 Hz to 30 Hz.
 7. The method asrecited in claim 1, wherein the static threshold value corresponds toprogrammed interpolation points within a static characteristics curve.8. The method as recited in claim 1, wherein, for the purpose of anoptimal receiver, a signal processing including an I/Q demodulation anda cross-correlation of an I and a Q channels using; an idealtime-shifted and a Doppler-shifted replica of a transmit signal isimplemented, and wherein an amount is formed from a sum of squares ofthe cross-correlation of the I and Q channels, the amount being scaledby an auto-correlation of the replica.
 9. The method as recited in claim1, further comprising: amplifying the receive signal; filtering thereceive signal by a bandpass filter with a mid-frequency on one of aresonant frequency and a transmit frequency of a transducer and with apass-bandwidth that is proportional to an inverse value of the burstlength; demodulating the envelope by a double-way rectifier and alow-pass filter, before the interference-level sample is ascertainedtherefrom.
 10. The method as recited in claim 1, wherein: theascertaining of the interference-level sample is performed during a timeinterval in which no reflected measuring signal from a target objectoccurs.
 11. The method of claim 1, wherein the ascertaining of theinterference-level sample is performed only during a time interval inwhich no reflected measuring signal from a target object occurs.
 12. Adevice for adapting a threshold of a detection device, comprising: atransmit/receive device for transmitting a transmit burst at apredefined pulse-repetition rate and a predefined signal frequency, andfor receiving a receive signal; a signal-processing device forprocessing the receive signal; a detection device for ascertaining areceive-signal magnitude as a noise-level sample from an envelope of thereceive signal after a first predefined time duration starting with thetransmit burst and within a predefined second time duration prior to anext transmit burst; a threshold generation device generating a staticthreshold value having a predefined time characteristic; a summingdevice generating a dynamic threshold value by summing the noise-levelsample with the static threshold value; and a comparator that comparesthe envelope of the receive signal to the dynamic threshold value andgenerates an output indicating that an object has been detected when theenvelope exceeds the dynamic threshold value.
 13. The device as recitedin claim 12, wherein: the device is incorporated in an ultrasonic systemfor detecting surroundings of a vehicle.
 14. The device as recited inclaim 12, wherein the transmit/receive device includes a transducer foremitting the transmit burst having a signal frequency between 40 KHz and100 KHz and for receiving an analog acoustical receive signal.
 15. Thedevice as recited in claim 14, wherein the transmit burst has a pulselength of 150 μs to 500 μs and a pulse-repetition rate of 10 Hz to 30Hz.
 16. The device as recited in claim 12, wherein the detection devicehas a peak-value rectifier for ascertaining the receive-signal magnitudeas a function of a release by a timer.
 17. The device as recited inclaim 12, wherein the signal-processing device includes at least anamplifier, a band-pass filter device, a demodulation device, a two-wayrectifier, and a low-pass filter, to produce a demodulated receivesignal.
 18. The device as recited in claim 17, wherein the staticthreshold value is output by the threshold generation device in responseto a clock pulse.
 19. The device as recited in claim 18, furthercomprising: a comparator for generating a digital detection signal bycomparing the demodulated receive signal to the dynamic threshold value.20. The device of claim 12, wherein the detection device performs theascertaining of the noise-level sample only during a time interval inwhich no reflected measuring signal from a target object occurs.
 21. Amethod, comprising: transmitting an ultrasound signal at a predefinedpulse-repetition interval, a predefined burst length, and a predefinedsignal frequency; receiving a reflected signal; processing the reflectedsignal to generate an envelope of the reflected signal; sampling theenvelope at a predefined time after each ultrasound signal transmission;during a time interval in which no useful signals corresponding to anobject to be detected contribute to the reflected signal, applying anamplitude detection procedure to the sampled envelope, therebygenerating an interference signal; generating a dynamic threshold bysumming the interference signal with a static threshold value having apredefined time characteristic; comparing the envelope to the dynamicthreshold; and generating an output indicating that an object has beendetected when the envelope exceeds the dynamic threshold.
 22. The methodof claim 21, wherein the amplitude detection procedure is performedusing a peak-value rectifier in which a hold-time constant is longerthan the pulse-repetition interval, a load-time constant is longer thanthe burst length, and a temporal-acquisition duration is longer than theload-time constant.